Filament for incandescent electric lamps and the like



F. S. GOUCHER.

FILAMENT FOR INCANDESCENT ELECTRIC LAMPS AND THE LIKE.

APPLICATION FILED NOV. 25, 1921.

Inventor.

Patented Nov 28, 1922..-

UNITED STATES FREDERICK SHAND GOUGHER, OF EAIMMERSMITH, LONDON, ENGLAND.

FILAMENT FQR INCANDESCENT ELECTRIC LAMPS AND THE LIKE.

Application filed November 25, 1921. Serial No. 517,679.

To all whom it may concern:

Be it known that I, FREDERICK SHAND GOUOHER, Ph. D. of ResearchLaboratories of The General Electric Company Limited, Brook Green,Hammersmith, London, W. 6, England, a British subject, have inventedcertain new and useful Im rovements in Filaments for Incandescentlectric Lamps and the like, for which I filed an application for apatent in Great Britain on October 28, 1920, Serial No. 30,552 of 1920,and of which the following is a specification.

This invention relates to filaments, for use in incandescent electriclamps and the like, which can be deformed and subjected to prolongedhigh temperatures without becoming brittle. v

The wire of such filaments will hereinafter be referred to as stablewire;

It is well known that the crystal grains of metal become elongatedduring drawing into wire. When the Wire is used as the filament of anincandescent electric lamp, the crystal grains tend to re-arrangethemselves; sometimes an intercrystalline-boundary develops whichextends across the greater part or the whole of the cross-section of thewire, and may become! a source of mechanical weakness. It is also knownthat filaments formed from squirted threads composed of fine crystalgrains, when heated for a sufiicient time to a sufliciently hightemperature, tend to develop a structure of much larger grains of unevensize.

In the specification of British Patent No. 16620 of 1914, a method ofpreventing such re-crystallization with its attendant mechanicalweakness is described. The method proposed consists in passing thefilament through a short zone of very high temperature at a speed equalto or less than the speed of crystal growth. It is stated therein thatby the treatment the wire is converted into a single crystal or severalsuch crystals arranged in juxtaposition -or in the form of a bundle.Wire in which such structural change has been effected is stable wire inthe sense referred to above. It has now been found, however, that wirewhich has not neeessarily the single-crystal structure may be stableunder certain conditions. Thus, wire may be rendered stable by theprocess according to this invention without assuming the single-crystalform and, furthermore, it is found that wire may be rendered stable ifits crystalline structure is made such that the crystal boundaries areinterlocked to a sulfielent extent.

The present invention consists in a process for rendering a wire stableby first annealing the wire, then subjecting it to a treatment WhlChsets up a suitable strain in the metal, and subsequently heating thewire to a temperature sufiiciently high for crystal growth to take placerapidly, the heating being carried out so that the region in which thereis a smtable temperature gradient travels along the wire at a relativespeed equal to or less than the speed of crystal growth. One method ofproducing the necessary strain in the metal is to. pass the wire througha die or series of dies at the temperature usually employed inwire-drawing the metal. Other methods of treatment which produce acompression in the wire will also serve the purpose, but the invention'is not limited to any particular method of producing the necessarystrain. Furthermore, according to the invention a stable wire isproduced which owes its properties to the fact that ,theboundaries ofthe crystals are such that the crystals eifectively interlock, as hasbeen deteradjacent but separate crystals; whilst the.

boundary between them, shown by the irmined .by microscopic examinationof such regular line 0., has such a form that the crystals areeffectively interlocked and can form part of a stable wire.

In order that the invention may be clearly understood and readilycarried into effect, an example of the improved process applied to drawntungsten wire of a particular diameter will now be more fully described.

A. tungsten wire of 0.043 mm. diameter and containing about 0.6 per centof thoria is subjected to an annealing treatment in a known manner forthe purpose of modifying thefibrous structure of the wire resulting fromthe normal drawing operations in reducing the diameter to the requiredsize. The annealing treatment of this wire consists-in heating it in areducing atmosphere to about 1600 for about one second.

The annealing treatment may, however, be

varied as to temperature and time of treatment according to the size,composition, and

extent of the previous mechanical treatment 1 of the wire.

The diameter is thereafter reduced from 0.0%?) mm to 0.04 mm.,by'drawing through a die of the usual kind, the temperature of the wirebeing maintained during the drawing at about 300 C. In this manner, a'suitable strain is introduced in the wire. I

i The wire is next subjected to a heated zone treatment in a knownmanner whereby each portion of the wire is succemively passed through azone of very high temperature, the zone having a very steep temperaturegradient substantially as described in the challer U.-S. Patent No.1,256,929. This heat treatment is arranged as a continuous process, thehigh temperature being producd by passing an electric current throughthe wire; the wire is automatically fedat an even speed of between oneand two metres per hour through the zone arranged between two contactsin a reducing atmosphere hydrogen or the like, thereafter emerging in acooled condition and being wound on to a reel. The heated zone may beabout one centimetre long. Nospecial precautions are required in coolingthe wire and no further mechanical treatment such as drawing isnecessary.

ent invention is applicable to filaments of squirted threads and of allkinds of drawn wire,it is particularly useful for producing stable wirefrom drawn wire in cases where theabove-mentioned single-crystal processis not efl'ective; It may also be applied to produce a stable wire whichconsists of a single crystal or several such crystals arrangedinjuxtaposition orin the form of a bundle and does not owe its stabilityto the fact that the crystals are eflectively interlocked. The inventionis useful in producing stable wire from drawn or otherwise mechanicallyworked tungsten wire with or without added materials such as thoria,alumina or silica.

I claim In the manufacture of tungsten filaments, the improved processwhich comprises annealing a wire, applying a suitable strain to thewire, and then passing the wire through a zone of steep temperaturegradient.

FREDERICK SHAND GOUCHER.

